All enveloped viruses exploit the cellular secretory pathway for biosynthesis of their membrane proteins. The best studied enveloped viruses assemble by budding from the plasma membrane, where their membrane proteins accumulate. Assembly of enveloped viruses at intracellular membranes is less well understood, although these viruses must also accumulate their membrane proteins at the budding site. Intracellular assembly of the avian coronavirus infectious bronchitis virus (IBV) in the cis Golgi network will be studied. Coronaviruses are ubiquitous in vertebrates, and in humans cause mild respiratory disease (responsible for about 20 percent of common colds). Coronaviruses are conveniently studied in cell culture systems, and are thus an ideal model for intracellular virus assembly. Understanding the intracellular assembly of enveloped viruses is important because several virus families that cause significant human disease assembly at intracellular membranes. These include Bun yaviridae and Flaviviridae. The long term goals of the proposed experiments are to elucidate the mechanism and advantages of intracellular assembly of enveloped viruses, and to identify unique strategies to interfere with assembly and infection by this subset of viruses. Specifically, the experiments are designed to test the following hypotheses: (1) the IBV RNA 3 proteins (3a, 3b, and E) play important roles in virus assembly and infection; (2) the IBV E protein has an additional function in infected cells to slow membrane traffic, allowing S to accumulate at the budding site and possibly preventing virus antigen presentation to the immune system; and (3) distinct envelope lipids (derived from the cis Golgi network) provide an advantage for the virus during subsequent rounds of infection by promoting fusion with susceptible cells.